Information About the Weathering of Rocks
Although rocks may seem very hard and tough, over time, like most substances, they are not immune to nature's forces. Rocks weather through any number of elements, and even mountains eventually find themselves shrinking. Yet the weathering of rocks can also create beautiful and even bizarre scenery, some of which is celebrated in America's National Park System.
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Kinds of Weathering
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There are two main ways that rocks weather. One is physical weathering, which wears down the rocks without changing their actual elemental makeup or identity. The second is chemical weathering, meaning that chemical agents actually break down the rock. Water can be an important part of both kinds of weathering.
Mechanical Weathering
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Mechanical or physical weathering can take on any number of forms. Temperature and water are the most common examples. Water can expand 9 percent when it freezes, so cycles of freezing and thawing will make cracks in rocks bigger over time, eventually wedging them apart.
Clays, called smectites, sometimes absorb water, expanding and causing rocks to split. Plant roots like to grow in a rock's cracks, which forces them apart as the plants get bigger. The intense heat of forest fires can cause the outsides of rocks to expand and crack parallel to the surface. When the rocks split from this process, it is called "spalling" off. -
Chemical Weathering
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Chemical weathering usually happens in the ground, because this is where water and rocks are constantly in contact. Gases and other solutions can also chemically alter rocks. Halite and calcite are two common minerals carried in water that dissolve rocks. When oxygen combines with iron-bearing silicate, the result can "rust" the rocks.
Biological Weathering
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A third kind of weathering is recognized, called biological weathering, which is when rocks are altered by living things. As noted above, plant roots splitting rocks is a kind of physical weathering, but this is also biological weathering. Lichen growing on rocks can also change them, and even animals and humans are active in altering rocks--one only has to look at the industries that extract materials like coal from the ground to see this in dramatic display.
Examples
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No matter how rocks are weathered, the results can be beautiful. Some weathering is unfortunate, as it wears away at monuments and gravestones. However, other examples, such as the Grand Canyon (caused by water and sediment scraping away at the walls), are national treasures. The Grand Canyon of Yellowstone National Park has walls that are stained a number of brilliant colors from the geysers that also dot the landscape. Utah is rich in interesting rock formations that are a result of weathering as well.
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References
- Photo Credit Grand Canyon image by Kimprebble from Fotolia.com
